In vehicles using electric traction motors, alternating current (AC) motor drives are used to provide a requested torque to the motor shaft. In practice, the amount torque produced by the motor is directly related (although not perfectly proportional) to the amount current provided to the motor. Therefore, by regulating and precisely controlling the input current to the electric motor, the amount of torque produced by the electric motor may be more accurately controlled. This is particularly useful, as in practice, the temperature and/or resistance of the electric motor is dynamically changing during operation. For example, by maintaining a constant current through the electric motor, the torque produced by the motor remains relatively constant, even as the resistance of the motor windings increases and/or decreases. Conversely, in the case of a constant voltage across the motor windings, as the resistance of the motor windings increases and/or decreases, the current through the electric motor decreases and/or increases, thereby changing the amount of torque produced by the electric motor at the constant voltage level.
In many systems, the input motor current is not directly controlled. For example, many electric motors are operated using pulse-width modulation (PWM) techniques in combination with an inverter (or another switched-mode power supply) to control the voltage across the motor windings, which in turn, produces the desired current in the motor. In response to a requested torque (or commanded torque), most prior art systems determine a desired input motor current for producing the requested amount of torque and utilize a closed loop control system to control the current through the motor windings and thereby regulate the amount of torque produced the motor (known as vector control or field oriented control). One or more sensors are used to obtain the actual motor current, which is then compared to the desired input motor current. Based on the outcome of the comparison, the PWM commands for the inverter are adjusted to increase and/or decrease the voltage across the motor windings, such that the actual measured motor current tracks the desired input motor current.
When a current sensor used to measure the motor current does not accurately measure the motor current, these closed-loop control systems are degraded and motor control is therefore compromised. For example, without accurate motor current information, the control system may cause the motor to produce insufficient torque, excessive torque, or varying or oscillating amounts of torque. In conventional prior art three-phase electric motor drive systems, each phase of the electric motor has an associated current sensor. Assuming balanced three-phase operation, the sum of the individual phase currents should equal zero at any time. In this regard, when the sum of the phase currents is not equal to zero, the system may identify an error of one of the current sensors and take preventative measures.
In the case of a balanced three-phase electric motor, it is possible to control the electric motor by only measuring the current in two of the three phases, and calculating the third phase current based on the balanced three-phase relationship. It is important to detect errors from one of the current sensors, however, in a system having only two current sensors, an error occurring in one current sensor leaves the system with no redundancy and the prior art methods that utilize three current sensors cannot be used to identify a current sensor error.